Transmission lines connected to an antenna or the like are generally unbalanced transmission lines, while transmission lines connected to a high-frequency circuit, such as a semiconductor IC, are balanced transmission lines. Accordingly, when connecting the unbalanced transmission line and the balanced transmission line, a balun transformer that mutually converts an unbalanced signal and a balanced signal is inserted between these lines. In this case, the unbalanced signal means a single ended signal with a fixed electric potential (such as a ground electric potential) as a reference, and the balanced signal means a differential signal.
A balun transformer using a spectacle-shaped core as described in Japanese Patent Application Laid-open No. H11-135330, and a balun transformer using a toroidal core as described in Japanese Patent Application Laid-open No. H8-115820 are examples of general balun transformers. However, there is a problem in the balun transformer using the spectacle-shaped core or the toroidal core in that not only it has a comparatively large overall size, but also it poses difficulties in the automation of the winding operation of a winding and in surface mounting.
Meanwhile, a balun transformer using a drum-shaped core as described in Japanese Patent Application Laid-open No. 2005-39446 has advantages that downsizing is easy and is suitable for the automation of the winding operation of a wiring and for surface mounting.
In the balun transformer using a drum-shaped core, however, its characteristics are greatly changed depending on a winding method of a secondary winding, and thus it is difficult to obtain a good high-frequency characteristic. Particularly in the high frequency area, it is difficult to obtain a good amplitude balance (amplitude balance in the balanced signal) and phase balance (phase balance in the balanced signal).